1. Field of the Invention
The invention is directed to a surge arrestor having a middle electrode and at least one outer electrode. An electrically conductive spring clip is secured to the middle electrode and exerts a spring force on the outer electrode.
2. Description of the Related Art
Surge arrestors of the type previously described are usually employed for securing telecommunication devices against briefly occurring over-voltages as resulting, for example, from lightning strikes. By triggering the surge arrestor, the outer electrode is shorted to the middle electrode by an arc. As soon as the occurrence of the over-voltage has ended, the arc vanishes and the gap between middle and outer electrode functions as an insulator again.
Arrestors can be equipped with additional functions in order to be able to maintain the previously described protective function even given an outage of a surge arrestor. Mechanisms are known in this context for securing the arrestor given a thermal overload (fail safe) in which a fusible element of solder material or an insulating foil as well is arranged between the spring clip and the outer electrode; this arrangement, given an excessively high temperature, releases the movement of the spring clip which then bridges and, thus, shorts the gap of the arrestor between middle electrode and outer electrode.
Another fault that may occur in an arrestor is the loss of tightness of the arrestor, this resulting in that the trigger voltage of the arrestor rises greatly. The arrestor then no longer triggers at the original trigger voltage and can thus also no longer generate heat that activates the fuse mechanism. In order to also secure the protective function in this situation, the arrestors can be additionally equipped with a fuse given looseness (vent safe). Each spark gap of the arrestor has an additional voltage-limiting component part connected in parallel to it. This can be a varistor or a semiconductor (for example, a break-over diode). This assures that the protective function is preserved even given a malfunctioning or leaking arrestor since, in this case, either the additional, voltage-limiting component part itself protects, shorts the arrestor, or triggers a thermal short-circuit mechanism due to heating.
The highest demands made of the fault-protection mechanisms arise given a leaking arrestor. For example, the American specification Telcordia 1361 prescribes a test in which a leaking 3-electrode arrestor is applied to an alternating voltage of 1000 V at which a maximum current of 30 Amperes per break can flow. In one arrestor version that envisions the use of varistors as voltage-limiting component parts, switching powers of 30 kW per break must be governed. This high electrical power necessarily leads to sparking and burn-up due to arc that harbors a fire hazard sine the arrestor are usually installed in a plastic housing.
U.S. Pat. No. 5,388,023 discloses arrestors of the species initially described in which a fusible element is arranged between the spring clip and the outer electrode. In the normal operating case, the fusible element must prevent a short circuit and, accordingly, must be fashioned in a solid manner with a minimum thickness. In case of a fault, the fusible element releases the electrical contact between the spring clip and the varistor or the outer electrode. This fusible element has a relatively large mass to be fused, as a result of which the transition of the fusible element from the solid into the molten phase lasts a long time and, accordingly, the triggering of the melt fuse is delayed. The risk of sparking thus rises greatly.
It is therefore an object of the present invention to provide a surge arrestor that provides a fast trigger mechanism in case of a fault.
This object is inventively achieved by a surge arrestor, comprising a middle electrode; an outer electrode; an electrically conductive spring clip secured to the middle electrode and configured to exert a spring force on the outer electrode; an electrical component part arranged between the spring clip and the outer electrode, the electrical component part being non-conductive at a trigger voltage of the surge arrestor and configured to generate heat given a flow of current; a fusible mass; a spacer element; and an electrically conductive contact element that is secured to the spacer element with the fusible mass and that is spaced from the outer electrode, the spring clip lying against the contact element, and the contact element being pressed against the outer electrode by the spring clip when the fusible mass melts. Advantageous developments are described below.
The invention provides a surge arrestor that comprises a middle electrode and at least one outer electrode. An electrically conductive spring clip is secured to the middle electrode and exerts a spring power on the outer electrode. An electrical component part that generates heat given a flow of current is arranged between the spring clip and the outer electrode. The spring clip lies against an electrically conductive contact element that is secured to a spacer element by a fusible mass and that is spaced from the outer electrode. The fusible mass adheres both to the contact element as well as to the spacer element. When the mass melts, the contact element is pressed against the outer electrode by the spring clip.
The inventive surge arrestor has the advantage that the isolation between the spring clip and the outer electrode required in the normal operating case of the arrestor is effected by the spacing of the contact element from the outer electrode by the spacer element. The fusible mass is only required in order to secure the contact element to the spacer element and can therefore be provided in a slight amount that need only assure that the contact element is held by the spacer element.
The electrical component part thus insulates at the trigger voltage that the arrestor has in normal operation.
In an advantageous embodiment of the invention, the electrical component part can be a varistor. Given standard, static trigger voltages of approximately 350 V of arrestors that are present during normal operation, such a varistor still has a high impedance of approximately 0.5 Mxcexa9, so that it practically represents an insulation between the spring clip and the outer electrode. In case of over-voltage, however, the resistance of the varistor is diminished, so that the protective function can be assumed by the varistor in case the trigger voltage of the arrestor rises to extremely high values ( greater than 1000 V) due to a fault in the arrestor. Over and above this, a varistor is an electrical component that generates heat when current flows and can itself therefore trigger a thermal protective mechanism.
In another embodiment of the invention, however, the component part can be a semiconductor component.
In one embodiment of the surge arrestor, the spring clip and the contact element are two different component parts, where the spring clip lies against the contact element and thus presses it against the outer electrode.
In another embodiment, the contact element is integrated as one piece in the spring clip. Here, too, the spring clip lies against the contact element since contact element and spring clip directly adjoin one another.
In one embodiment of the invention, the spacer element is in the form of a pin. The contact element comprises a disk that has a hole into which the spacer element projects.
This embodiment of the invention has the advantage that it can be especially easily and simply manufactured. The fastening of the contact element to the spacer element can, given a corresponding dimensioning of the pin or hole, be produced with a very slight amount of fusible compound, this yielding the advantage of a fast trigger mechanism.
In another embodiment of the invention, the spring clip likewise comprises a hole through which the spacer element projects when the compound has melted. This avoids impeding the trigger mechanism due to a mechanical contact between the spring clip and the spacer element. Depending on the fashioning of the spacer element, the spacer element can already project through the hole in the spring clip even when the compound has not yet melted.
In another embodiment of the invention, the spacer element comprises a taper in a section lying between the contact element and the outer electrode. This taper has the advantage that, when the compound melts, the motion of the contact element along the pin-shaped spacer element is not impeded by excessively slight spacings between the inside edge of the hole of the contact element and the spacer element, this promoting the design of a very fast protective mechanism. As a result of the hole in the contact element through which the spacer element projects, the spacer element can also be advantageously used for guiding, fixing and aligning the contact element.
In another advantageous embodiment of the invention, the electrical component part is arranged between the outer electrode and the spacer element. As a result, an arc arising in the electrical component part can be prevented from penetrating toward the outside given overload of the component part.
In an advantageous embodiment of the invention, the fusible mass can be fashioned as solder. In conjunction with solderable materials for the contact element and the spacer element, a very simple connection between contact element and spacer element is thus possible. Moreover, the tin alloys employed for solder assure that the connection between the contact element and the spacer element is quickly undone given adequate heat.
In another embodiment of the invention, the contact element comprises a disk that has its disk edge provided with a collar extending in the direction toward the outer electrode. Such a collar, within which the electrical component part is especially advantageously arranged, can additionally effectively reduce the arc-over of sparks from the electrical component part to conductive objects outside the electrical component part. Such a disk with collar can be especially advantageously implemented in the form of a cap.
The contact space between the varistor and the electrically conductive spacer element can also be covered with the assistance of the collar. This contact space is required as long as the electrical component part assumes the voltage-limiting function. The varistors usually employed as an electrical component part, however, cannot handle the same electrical power or the same high currents as the surge arrestor. Given longer-lasting over-voltages or given extremely high over-voltages, the varistor is very quickly destroyed by a breakdown, resulting in the heat triggering the fuse mechanism. Maximally, thus, the varistor represents a short-duration protection given smaller over-voltages or over-currents.
In another embodiment of the invention, an outside surface of the electrical component part can be covered with a shrink hose. This reduces sliding discharges and, likewise, the formation of sparks at the outside of the electrical component part.
A shrink hose can also be arranged on the outside surface of the collar, this shrink hose additionally covering the contact space formed between the electrical component part and the spacer element.
Such a shrink hose, moreover, reduces the delivery of oxygen to the contact element and, thus, the risk of sparking. Furthermore, this also effectively diminishes the risk of a lateral short-circuit between the contact element and the outer electrode.
A shrink hose at the outside surface of the collar is significant when, in another embodiment of the invention, the outer electrode comprises a ring at its edge that is composed of an iron-nickel alloy. The outer electrode projects very far in the direction of the contact element through this ring, the risk of a lateral short-circuit increasing as a result and the arrangement of a shrink hose at an outside surface of the collar being thus especially advantageous.
The ring at the outer edge of the outer electrode has the advantage that the different coefficients of thermal expansion that represent a disturbing factor when soldering the outer electrode onto the small ceramic tube usually employed as a component body can be compensated by occupying that side of the outer electrode lying opposite the small ceramic tube with a material that is similar to the small ceramic tube.